This application claims the priority of Korean Patent Application No. 2004-25246, filed on Apr. 13, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a quantum-key distribution method and, more particularly, to a quantum-key distribution method of a quantum cryptographic system in a network used by a plurality of users or groups.
2. Description of Related Art
Quantum-key distribution is a field of quantum cryptography which is a cryptographic system using quantum mechanics. Since a conventional cryptographic system is generated on the basis of mathematical problems where the calculation is known to be difficult, the security is not guaranteed as rapid progress is made in calculation abilities. However, quantum-key distribution is a cryptographic system using a one-time. The security of this cryptographic system is guaranteed by quantum-mechanical properties.
However, since a conventional quantum-key distribution is a cryptographic system based on a transmission of a quantum state, the conventional quantum-key distribution has mainly employed a protocol used between both users who trust quantum channels. While there have been patents concerning a method of actually realizing the aforementioned protocol, there is no patent concerning the protocol itself. Since the conventional quantum-key distribution should be based on trust in the channels used by each other, a quantum-key distribution network protocol simply connecting between both users trusting in each other has been mainly offered.
A paper entitled “Quantum cryptographic network based on quantum memories” in “Physical Review A” in 1996 discloses a quantum-key distribution method totally different from conventional methods. A protocol on a network using the method is also disclosed in this paper.
In contrast to the conventional quantum-key distribution method where an Einstein-Podolsky-Rosen (EPR) pair is first shared and then measured, in the quantum-key distribution method disclosed here, each of users, Alice and Bob, trying to distribute keys, chooses any one of four states used in the Bennett Brassard (BB84) and transmits the chosen states to a center, and then the center stores the transmitted states in a quantum memory, measures the states transmitted from the two users, and notifies the two users of the measurement results, whereby each of the users detects states transmitted from the other side on the basis of states transmitted from his own side, and acquires a key if axes are equal.
The center assists and interlinks two users to distribute a key. Each of the users transmits a required number of quantum states to the center, and the center stores the quantum states. Two users notify the center that they desire to share a key, and the center chooses and measures quantum states for the two users and notifies the two users of the measurement result. However, in this method, a quantum memory is required for a long-time storage and an efficiency for the used states is very low.
In a paper entitled “Quantum key distribution relied on trusted information center” in “Los Alamos e-print quant-ph” in January 2000, a quantum-key distribution between two users is controlled by a center, but the center only assists two users to distribute a key.
In other words, in a case where each of a center and two users A and B distributing a key has a particle using Greenberger-Horne-Zeilinger (GHZ) 3-qubit state, the center measures its own qubit and notifies the two users of the measurement result, and the users A and B detect encryption keys owned by each other on the basis of their own measurement results depending on the result of the center.
A paper entitled “Conditional efficient multiuser quantum cryptography network” in “Physical Review A” in January 2002 discloses a quantum-key distribution protocol using three nonorthogonal states. In addition, the paper describes that a quantum cryptography network is possible by using a space optical switch.
According to this paper, three users including a center select two probabilities, prepare states depending on the selected probabilities, and make two kinds of measurements on each of transmitted photons depending on the selected probabilities. That is, the security is based on the probable selection and measurement. In addition, the paper discloses a method of establishing a cascaded quantum cryptography network using the quantum-key distribution protocol and the space optical switch. However, this method can be employed when only a center can control a number of switches.
The methods disclosed in the aforementioned papers are available only if two users trust quantum channels since the methods are implemented under the basic assumption that the states used by the users distributing keys can be correctly transmitted at desired places. Accordingly, all the conventional quantum-key distribution network protocols cannot be applied to a typical network system established under the assumption that the network system is used by an unspecified number of users. In addition, the conventional quantum-key distribution network protocols cannot guarantee the unconditional security which is a basic feature of the quantum cryptography.